1. Field of the Invention
The present invention relates to a sequencing system and method for varying the output of airbag inflators, and more particularly, to a system and method for sequencing two or more initiators in an airbag inflator.
2. Description of the Related Art
Current airbag inflation technology employs rapidly expanding gases to inflate an airbag in order to reduce occupant injury. However, to prevent injury to an occupant, especially a child, the airbag should initially deploy slowly such that the occupant is pushed backward into the seat. Then after an appropriate delay, if necessary, the bag can be expanded more rapidly to further protect the occupant. A graph of pressure vs. time performance that will produce an initial gentle but firm inflation followed by a rapid inflation is commonly known as an S-curve, as illustrated in FIG. 1.
In order to produce such S-curve parameters variable stage inflators which tailor the mass flow and total gas output of the airbag inflation device are used to vary the level of inflation.
One manner of varying inflation levels is to employ separate chambers within the inflator that are capable of being activated either individually or in sequence by an individually associated initiator or squib to affect the desired gas flow rates. See U.S. Pat. No. 4,998,751, assigned to the assignee of the present invention. The preferred method of activation is by an electrical signal that is sent through a squib in each of the separate chambers. One disadvantage with such a manner of activation is the complexity and expense of the external electronic hardware required to support the proper firing of the various squibs.
U.S. Pat. No. 4,213,635 discloses a dual combustion chamber airbag system wherein a delayed ignition signal generator disposed in a first chamber in response to the pressure therein activates a second ignitor to increase inflation. The generator is a mechanical device located within the inflator whose delay is determined by the tension of a spring or frictional force of an activator. Thus, not only are two combustion chambers required, but the effectiveness of the generator is dependent on a potentially unreliable delay mechanism.
Another method of varying the rate of inflation of an airbag is disclosed by U.S. Pat. No. 3,972,545, assigned to the assignee of the present invention. The inflator is divided into two chambers by a partition. Each of the chambers contain gas generant material ignitable by an individually associated initiator or squib. An impact sensor determines whether one or both of the squibs are fired on impact, and hence, the rate at which the airbag is filled with gas. If the impact is severe, both squibs will be fired. However, if the impact is less severe, only the downstream squib will be fired. The combustion of the downstream squib will proceed upstream through the partition to ignite the gas generant in the upstream chamber, the downstream squib in effect igniting both chambers of generant material. Thus, the electrical hardware is not designed to fire the second squib after a predetermined delay.
U.S. Pat. No. 4,358,998, assigned to the assignee of the present invention, discloses an igniter assembly which is operable to cause the combustible gas generant material in a single combustion chamber to ignite in a progressive manner so as to effect inflation of the gas bag slowly initially but more rapidly later as inflation progresses. The igniter assembly includes a solid propellant disc between two portions thereof whereby the gas generant material disposed around one portion is ignited immediately, and then, after burn through of the disc, the gas generant material disposed around the other portion is ignited. The time required to burn through the disc is the delay time.
Although the above discussed prior art provides a controlled inflation rate for an airbag, there is still a need for a less complex and inexpensive system for a variable inflation rate.